Alcohols Oxidized Without Solvent

A catalyst consisting of gold-palladium alloy nanocrystals supported on titanium dioxide significantly speeds up the oxidation of alcohols to aldehydes under mild, solvent-free conditions (Science 2006, 311, 362).

"The bimetallic catalyst oxidizes alcohols, principally primary alcohols, to aldehydes with high selectivity, using oxygen at low temperature and without the use of a solvent," says Graham J. Hutchings, chemistry professor at Cardiff University, in Wales, who led the team that developed the catalyst.

Selective oxidation of alcohols to aldehydes using oxygen instead of expensive and toxic oxygen donors, such as chromate and permanganate, is potentially important for the synthesis of fine chemicals. However, the catalysts that have been investigated to date for this green process have proven relatively inactive for the oxidation of primary alcohols.

"Our catalyst is over 25 times more active than the previous best catalysts," Hutchings says.

The team showed, for example, that the Au-Pd/TiO₂ catalyst is highly active for the oxidation of benzyl alcohol with oxygen at 80 ^oC. "We observed selectivities for benzaldehyde of over 96%," Hutchings adds. "The catalyst also gives high turnover frequencies, up to 270,000 turnovers per hour."

Scanning transmission electron microscopy combined with X-ray photoelectron spectroscopy revealed that the Au-Pd nanocrystals consist of a gold-rich core and a palladium-rich shell. The researchers argue that gold acts as an electronic promoter for palladium and that the active catalyst's surface is significantly enriched in palladium.

Hutchings and coworkers also showed that turnover frequencies were the same when air was used as the oxidant in place of O₂. The results demonstrate that, in principle, air can be used for the industrial oxidation of primary alcohols to aldehydes, the team suggests.